In the v3 prerelease, if you want to force map mode for all source generated formatters, you have to set a property on the GeneratedMessagePackResolverAttribute:

This feels... odd. Both because it changes formatter behavior but is on a resolver attribute, and because it requires the user to declare a partial class for the source generated resolver, which otherwise the user almost never has to know about.

How about we move this property to a new assembly-level attribute where this and potentially other formatter-defaults (such as the one proposed in #1953) can be specified?